Experimental Study On Chemical Leaching Of Cr(Ⅵ) And Pb Compound Contaminated Soil Using Carboxymethyl Chitosan

The soil polluted by compound heavy metal is becoming serious increasingly.The compound pollution of lead（Pb）and chromium（Cr）cannot be ignored in the fields of metal smelting,chemical manufacturing,electroplating and some farmland soils.Currently,commonly used methods for remediation Pb and Cr contaminated soil include chemical reduction,solidification and stabilization,and chemical leaching.It could cause some problems such as low removal efficiency of heavy metals,large impact on soil characteristics,and serious secondary pollution in the process of using chemical leaching method to repair Cr（Ⅵ）and Pb compound contaminated soil.The purpose of this subject is to study the characteristics of Cr（Ⅵ）and Pb compound contaminated soil and select efficient and environment-friendly eluent to construct a new chemical leaching method for heavy metal compound contaminated soil.In this paper,the characteristics of Cr（Ⅵ）and Pb compound contaminated soil were characterized by XRF and XRD.The existing forms of Cr and Pb in Cr（Ⅵ）and Pb compound contaminated soil were measured by chemical analysis method.The physicochemical properties and pollutant forms of Cr（Ⅵ）and Pb compound contaminated soil were analyzed.Based on the theoretical analysis of the removal of Pb Cr O₄,Cr（Ⅵ）and Pb²⁺in soil,CMCS ware selected as the eluent to repair Cr（Ⅵ）and Pb compound contaminated soil.Secondly,CMCS was used as the eluent to study the remediation properties of CMCS and the influences of leaching time,p H,initial concentration of CMCS when repairing Cr（Ⅵ）and Pb compound contaminated soil by chemical leaching through batch experiments.Finally,the complexation of CMCS with Cr（Ⅵ）and Pb²⁺,the solubilization of CMCS on Pb Cr O₄ and the reduction of Cr（Ⅵ）by CMCS was studied by batch experiments.The infrared spectra of CMCS and its complex with Cr,Pb and the changes of the existing forms of Cr,Pb in the soil before and after leaching were analyzed.The remediation mechanism of Cr（Ⅵ）and Pb compound contaminated soil by CMCS leaching was proposed.The main research conclusions are as follows:（1）There were significant differences in mineral composition and chemical forms of heavy metals in Cr（Ⅵ）and Pb compound contaminated soil and Cr（Ⅵ）and Pb contaminated soil.In terms of mineral composition,the content of Pb Cr O₄ in the Cr（Ⅵ）and Pb compound contaminated soil was higher than that in Cr and Pb contaminated soil.The content of Cr（Ⅵ）and Cr（III）in the Cr（Ⅵ）and Pb compound contaminated soil was68.6%and 31.4%,respectively,while the content of Cr（Ⅵ）and Cr（III）in the Cr（Ⅵ）-contaminated soil was 92.7%and 7.3%,respectively.In terms of the existing forms of heavy metals,the content of residual Pb in the Cr（Ⅵ）and Pb compound contaminated soil was higher than that in the Pb-contaminated soil.In the Cr（Ⅵ）and Pb compound contaminated soil,the Cr mainly existed in the form of residual and bounded by Fe-Mn oxide,while the Cr mainly existed in the form of residual in Cr（Ⅵ）-contaminated soil.（2）CMCS had good chemical leaching remediation performance for Cr（Ⅵ）and Pb compound contaminated soil,Pb-contaminated soil and Cr（Ⅵ）-contaminated soil.When CMCS was used as the eluent,the removal rates of Cr（Ⅵ）and Pb from Cr（Ⅵ）and Pb compound contaminated soil was 34.1%and 5.9%respectively,the removal rate of Pb from Pb-contaminated soil was 7.4%and the removal rate of Cr（Ⅵ）from Cr（Ⅵ）-contaminated soil was 53.7%.The removal effect was better than that of deionized water.（3）The leaching time,p H and initial concentration of CMCS had significant effects on the removal of heavy metals in Cr（Ⅵ）and Pb compound contaminated soil and Pb-contaminated soil by CMCS.The leaching time and p H had significant effects on the removal of heavy metals in Cr（Ⅵ）-contaminated soil,while the initial concentration of CMCS had no significant effect.When the CMCS concentration was 8 g/L,the removal rate of Pb in the Cr（Ⅵ）and Pb compound contaminated soil increased first,then decreased and tended to be stable.The removal rate of Pb from Cr（Ⅵ）and Pb compound contaminated soil reached the maximum value of 20%at 0.5 h.The removal rate of Cr（Ⅵ）in the Cr（Ⅵ）and Pb compound contaminated soil showed a trend of first rapid increase and then slow increase with time.The removal rate of Cr（Ⅵ）from Cr（Ⅵ）and Pb compound contaminated soil reached 99%at 24 h.The removal rate of Pb from Pb-contaminated soil increased first,then decreased and then tended to be stable with time,reaching the maximum value of 24%at 0.5 h.The Cr（Ⅵ）removal rate of Cr（Ⅵ）in Cr（Ⅵ）-contaminated soil showed a trend of first rapid increase and then slow increase with time,reaching 70%at 24 h.The increase of p H was beneficial to the removal of Cr（Ⅵ）and Pb from Cr（Ⅵ）and Pb compound contaminated soil,Pb-contaminated soil and Cr（Ⅵ）-contaminated soil by using CMCS.The removal rate of heavy metals from Cr（Ⅵ）and Pb compound contaminated soil and Pb-contaminated soil increased with the increase of the initial concentration of CMCS.However,the initial concentration of CMCS had no significant effect on the removal of Cr（Ⅵ）by leaching from Cr（Ⅵ）contaminated soil.（4）CMCS could complex with ionic states Cr（Ⅵ）and Pb²⁺,promote the dissolution of Pb Cr O₄ significantly and could reduce Cr（Ⅵ）under acidic conditions.The exsting forms of Cr and Pb in contaminated soil changed significantly before and after remediation.CMCS complexed Cr（Ⅵ）and Pb²⁺through amino,carboxyl and hydroxyl groups,and the complexing capacity of CMCS to Cr（Ⅵ）and Pb²⁺are 6.35 mg/g and11.33 mg/g,respectively.Compared with medium and acidic conditions,more Pb Cr O₄was dissolved in alkaline conditions by CMCS.When p H=13,the concentration of Cr（Ⅵ）and Pb in the aqueous solution was 106.5 mg/L and 0.36 mg/L respectively in the process of dissolving Pb Cr O₄ by CMCS.When p H=3 and the reaction time reached 48 h,the reduction ability of Cr（Ⅵ）by CMCS was 7.03 mg/g.CMCS had a good removal effect on the residual Pb,carbonate-bound Pb,residual Cr in Cr（Ⅵ）and Pb compound contaminated soil,the carbonate-bound Pb in Pb-contaminated soil and residual Cr in Cr（Ⅵ）-contaminated soil.